Field of the Invention
The invention relates to a process for operating a fuel cell installation and to a fuel cell installation for carrying out the process.
It is known that, during the electrolysis of water, the water molecules are decomposed by electric current into hydrogen and oxygen. In fuel cells, that process takes place in reverse. During the electrochemical combination of hydrogen and oxygen to form water, electric current is produced with high efficiency, and when pure hydrogen is used as a process gas, it is produced without the emission of pollutants and carbon dioxide. Even with technical process gases, for example natural gas or coal gas, and with air, or air enriched with O.sub.2 instead of pure oxygen, a fuel cell produces considerably less pollutants and less CO.sub.2 than other forms of energy production which operate by using fossil energy sources. The technical implementation of the fuel cell principle has given rise to a wide variety of solutions, and more precisely with different electrolytes and with operating temperatures of between 1000.degree. C. and 800.degree. C. Fuel cells are divided, depending on their operating temperature, into low, medium and high temperature fuel cells, which in turn differ through various technical embodiments.
A fuel cell block, which is also referred to as a "stack" in the specialist literature, is generally composed of a multiplicity of fuel cells stacked on one another.
A problem which arises in this case is that of humidifying and compressing the process gases before they enter the fuel cell block, since the enthalpy of vaporization needs to be provided for the humidification. Devices with membrane humidification or water injection after compression are known from the literature, for example from German Patent DE 43 18 818 C2. One aspect which cannot be ignored is, moreover, the overall size of the membrane humidifier, which is of the same order as that of the fuel cell block, and leads to an additional requirement for space. Both solutions are found to be cost-intensive.
Further, German Published, Non-Prosecuted Patent Application DE 42 01 632 A1 discloses a process for humidifying at least one reactant flowing to an acidic or alkaline fuel cell. That reactant is only separated from the same reactant flowing out of the fuel cell by a semipermeable membrane.
In addition, Published French Patent Application 2 349 221 describes a fuel cell installation in which two liquid ring pumps feed the gases into the fuel cells. The off-gases of the fuel cells are fed back in a circuit and separated from entrained potassium carbonate electrolyte as a result of the latter being transferred into the liquid ring by centrifugal force in the liquid ring pump.